The present invention relates to a method of manufacturing diaphragms of deposited asbestos, consolidated by a fluorinated polymer resin, which are of adjustable porosity and can be used in electrolysis cells, as well as the new diaphragms thus obtained.
Diaphragm cells for use in the electrolysis of salts have been known for a long time. See "Chlorine, Its Manufacture, Properties And Use," by J. S. Sconce, American Chemical Society Monograph, Series No. 154, page 105, (Reinhold Publishing Corp. New York). However, the mechanism of their operation is not fully known. The diaphragms, as is well known, are placed between the anodic and cathodic components of the cells and act as a filter between anolyte and catholyte.
In general, in order for a diaphragm to be adapted properly to electrolysis conditions, it must be uniform in dimensions and texture and withstand corrosion in acid or alkaline hot chlorinated medium. This diaphragm behaves like a porous medium permitting both the passage of the current with a small ohmic drop and the uniform flow of the electrolyte from one compartment of the electrolysis to another.
In the case of the manufacture of caustic alkali from sodium chloride, the flow of the hydroxide ions in the direction opposite to the flow of liquid which is responsible for the formation of chlorates must be reduced to a minimum. In order to decrease this reverse flow, one can control the diaphragm by simultaneously increasing its thickness and decreasing its porosity, but the permeability of this diaphragm must, however, be sufficient for the percolation to take place with a loss in pressure head which is compatible with the requirements of the system. For a given permeability of the diaphragm, there will correspond a loss in head which is a function of the rate of flow of liquid. It will be necessary to adapt the diaphragm to the given conditions of current density and operation of the electrolysis permitting the best possible compromise between the chemical efficiency (loss of chlorine by formation of chlorate) and the electrical efficiency (drop of voltage in the diaphragm and loss of current by Joule effect).
The combination of electrical, chemical and hydraulic conditions indicated above is combined satisfactorily in those diaphragms obtained by the mere deposition of asbestos fibers on the cathode from a bath, for a current density close to 15 amperes per square decimeters of surface of electrode and diaphragm. This method of deposition is, on the other hand, poorly adapted to the operating conditions of electrolysis cells which are subjected to a higher current density unless ohmic drops which prove to be economically prohibitive are to be tolerated. The mere depositing of the asbestos fibers can lead only to structures having porosities which are difficult to control. It furthermore has the drawbacks of unconsolidated structures, namely: (1) a swelling during electrolysis, which requires a minimum interpolar distance, (2) the difficulty in obtaining sufficiently small deposits of asbestos having a low ohmic drop, and (3) the unstable state of the resulting diaphragm which, after the starting up of the electrolysis and stabilization, only very poorly withstands the incidents of operation and changes in situ and in the cell.
For these reasons, the industry has turned recently to the production of porous plastic diaphragms. The principle involved is well known. It consists in producing a composite having a base of asbestos and a polymer which is inert towards the electrolyte, with the possible presence of a pore former which is decomposed at the end of the operation to produce the required porosity.
Numerous references are to be found relating to such diaphragms. Mention may be made more particularly to the following patents which employ techniques of compression preforming followed by fritting, or techniques of coagulation of the mixture or depositing of this mixture on a support.
Thus French patent No. 1,491,033 of Aug. 31, 1966, describes a process of manufacturing a porous diaphragm which consists in mixing a solid additive in particulate form into an aqueous dispersion of polytetrafluoroethylene in the presence of particulate inorganic fillers, then coagulating the dispersion, placing the resulting coagulum in sheet form, and finally removing the solid particulate additive from the sheet. The additive generally consists of starch or calcium carbonate and is removed at the end of the operation by immersing the resultant sheet in hydrochloric acid to dissolve the additive. This additive may also be a plastic polymer which is soluble in an organic solvent, or depolymerizable, or else evaporatable by heating the sheet. The particulate inorganic fillers which are suitable are barium sulfate, titanium dioxide or powdered asbestos. They are used in proportions of between 40 and 70% of the weight of polytetrafluoroethylene contained in the dispersion.
British patent No. 943,624 of Dec. 14, 1961, proposes a method of producing a filter material which consists in mixing polytetrafluoroethylene in powder form with an eliminatable powdered material, subjecting the mixture to preforming under high pressure, and then sintering the resultant shape at a temperature which does not affect the polymer, the powdered material being eliminated either by volatilization at the sintering temperature or by the addition of solvents in which it is solubilized.
German application No. 2,140,714 of Aug. 13, 1971 claims a process of manufacturing diaphragms having a base of inorganic fibers, particularly asbestos, which are bonded by a fluorinated resin. The membrane can be obtained by impregnating a paper or fabric, or else produced by the introduction of fibers into the resin suspension and shaping in accordance with a paper-making method. The sintering is then effected under elevated pressure.
All of these foregoing prior art techniques, however, have a number of drawbacks, namely:
1. Providing flat diaphragms only, either because the use of calendering or pressing makes any other shapes impossible, or that the initial suspensions, in particular when they are coagulated, do not have sufficient properties to permit homogeneous deposits on supports of complex shape.
2. Difficulties, in the case of membranes rich in polytetrafluoroethylene, in producing membranes of satisfactory mechanical properties (permitting large flow) and of good wettability.
3. Low percentage of voids is permitted in the diaphragm structure. In order to obtain good mechanical properties and excellent conservation of the cohesion during electrolysis, the quantities of pore-forming agents used are zero or low, namely, 200-300%, or less, by weight of material. Under these circumstances, the performances in the electrolysis of sodium chloride are not truly of interest -- rather large ohmic drop or low Faraday yield, resulting from the reduced porosity of the diaphragm.
Other prior art is also less than satisfactory. British patent No. 1,160,084, published July 30, 1969, discloses membranes and diaphragms produced from a matrix of a fluorocarbon polymer and a combustible fibrous substrate, such as of cellulose, which can be burned out of the matrix. The resulting product is porous in nature, due to the voids left by the burning of the cellulose. According to the patent asbestos in the diaphragm is to be avoided.
British patent No. 1,063,244, published Mar. 30, 1967 describes a porous medium which is unsuitable for use in electrolysis cells. It is comprised of a porous base, such as of paper, having fibers, such as of asbestos, adhered to the surface, with the aid of a polymeric binder.
It is, accordingly, an object of the present invention to provide a novel and improved method of producing diaphragms suitable for electrolysis cells.
It is also an object of the invention to provide and an improved method of producing diaphragms which are free from the deficiencies of the prior art.
The applicant has now discovered a process which is simple and interesting to carry out and which forms one of the objects of the present invention.